Computer systems in the prior art have employed a variety of devices and techniques to permit the communication between subsystems of a system. In general, a computer system includes subsystems such as central processing units, video display devices, keyboards, and other units generally referred to as terminals. Communication between such a keyboard and a terminal in the prior art involved two techniques.
The first technique, as best illustrated in FIG. 1, is the use of UARTs to transmit and receive data between the keyboard and the terminal. UART is an acronym for "universal asynchronous receiver and transmitter." It is a device that is capable of transmitting and receiving serial data and transforming the data into parallel data, or vice versa. Of the four wires which are provided to. interface the keyboard with the terminal, wires one and two permit communication between the keyboard and the terminal. The two remaining connections are used to connect the keyboard and the terminal to a power source and a ground, respectively. In operation, a keyboard microprocessor first scans the keyboard switch array to determine which key of the array has been depressed. Generally, the code that has been assigned to identify the particular key is then transmitted to the keyboard microprocessor via the keyboard UART. The terminal UART receives this code and transmits it to the terminal microprocessor. This code is transmitted across one of the data links such as wire one or wire two. The terminal microprocessor then stores this information or retransmits it to other devices that are connected to the terminal. Macy, U.S. Pat. No. 4,346,369, discloses some aspects of this first technique.
The alternative technique for communication in the prior art, as shown in FIG. 2, is the use of a plurality of data wires. In such a case, for example, a ten-wire interface is provided in the keyboard and a similar tenwire interface is provided in the terminal 14. In this instance, seven wires are used by the terminal, in essence, its microprocessor to address the keyboard switch array. An eighth wire is used to transmit data from the keyboard to the terminal. The remaining two wires are the power and ground wires. UARTs are eliminated in this alternative technique. Fowler, III, U.S. Pat. No. 4,408,191 and Amano et al., U.S. Pat. No. 4,470,038 describe some aspects of this second technique.
Each of these prior art techniques, however, is deficient in some aspects. For the first prior art technique, not only are UARTs relatively expensive devices but also a microprocessor is required in the keyboard. As for the second prior art technique, a ten-wire cable is both relatively expensive and cumbersome.